ES45CH02-Ward ARI 15 October 2014 8:51 The Phylogeny and Evolution of Ants Philip S. Ward Department of Entomology & Nematology, and Center for Population Biology, University of California, Davis, California 95616; email: [email protected] Annu. Rev. Ecol. Evol. Syst. 2014. 45:23–43 Keywords First published online as a Review in Advance on Formicidae, fossil record, long-branch attraction, process heterogeneity, August 15, 2014 convergence, divergence The Annual Review of Ecology, Evolution, and Systematics is online at ecolsys.annualreviews.org Abstract This article’s doi: Originating most likely in the early Cretaceous, ants have diversified to be- 10.1146/annurev-ecolsys-120213-091824 come the world’s most successful eusocial insects, occupying most terrestrial Copyright c 2014 by Annual Reviews. ecosystems and acquiring a global ecological footprint. Recent advances in All rights reserved Annu. Rev. Ecol. Evol. Syst. 2014.45:23-43. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 11/25/14. For personal use only. our understanding of ant evolutionary history have been propelled by the use of molecular phylogenetic methods, in conjunction with a rich (and still growing) fossil record. Most extant ants belong to the formicoid clade, which contains ∼90% of described species and has produced the most so- cially advanced and dominant forms. The remaining ants are old lineages of predominantly cryptobiotic species whose relationships to one another and to the formicoids remain unclear. Rooting the ant tree is challenging because of (a) a long branch separating ants from their nearest outgroup, and (b) heterogeneity in evolutionary rates and base composition among ant lineages. These factors will need to be given careful consideration in future phylogenomic studies of ants. 23 ES45CH02-Ward ARI 15 October 2014 8:51 INTRODUCTION Eusociality, a state of cooperation involving overlapping generations, collective care of the young, and reproductive division of labor, has evolved in relatively few animal groups, most notably in the Hymenoptera (ants, bees, and wasps) and the Blattodea (cockroaches) (Wilson 1971, Inward et al. 2007). By most measures the ants (Hymenoptera: Formicidae) represent the most successful of all such experiments in eusocial life. They have diversified into tens of thousands of species, colonized most of the world’s terrestrial ecosystems, and acquired multifarious ecological roles. Ants have a significant impact on populations of other organisms through their varied activities as scavengers, predators, seed harvesters, honeydew feeders, herbivores, and ecosystem engineers (Holldobler¨ & Wilson 1990, Folgarait 1998, Lach et al. 2009). In some tropical forests the biomass of ants exceeds that of terrestrial vertebrates severalfold (Fittkau & Klinge 1973), and ants are the dominant consumers of plant resources in the canopy of lowland rainforests (Davidson et al. 2003). Many ant species have developed symbiotic relationships with other life-forms, participating in a complex web of interactions with microbes, fungi, plants, and other animals (Holldobler¨ & Wilson 1990, Davidson & McKey 1993, Mueller et al. 2001, Russell et al. 2009). There are approximately 13,000 described species of ants (AntCat 2014), but the actual number of extant species may well be two or three times this. We are far from having a complete inventory at the species level: Numerous ant species remain undiscovered and/or undescribed, especially from tropical regions, and many of the larger ant genera are in a state of taxonomic disarray (Ward 2007). This review focuses on the major lineages of ants and summarizes our current understanding of their evolutionary history. In contrast to species-level taxonomy, progress on the higher-level phylogeny of ants has been rapid in recent years, although there are still some thorny issues remaining to be resolved. This is a fast-moving field, and with the increasing use of genomic data in phylogeny estimation (Lemmon & Lemmon 2013) we can expect to see further advances in our knowledge in the near future. HISTORICAL CONTEXT: ANTS AS MODIFIED WASPS Hymenopteran Heritage Ants are members of the order Hymenoptera, a large and successful group of haplodiploid in- sects, comprising wasps and their derivative cousins, the ants and bees. The oldest lineages of Hymenoptera are a series of broad-waisted wasps, the “Symphyta,” that are predominantly phy- tophagous and relatively species-poor (Huber 2009). Most hymenopteran diversity resides in a large clade known as the Apocrita, which arose within a subgroup of symphytans, probably in the Annu. Rev. Ecol. Evol. Syst. 2014.45:23-43. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 11/25/14. For personal use only. Triassic or early Jurassic (Rasnitsyn 1975, Ronquist et al. 2012). Characterized by the fusion of the first abdominal segment to the thorax and the presence of a strong constriction between the first and second abdominal segments, the Apocrita include an enormous variety of parasitoid and predatory wasps, as well as some plant-feeding taxa (Gauld & Bolton 1988). The reconfiguration of body segments in this group is associated with considerable dexterity of abdominal movement. This may have been of particular importance in one subclade of Apocrita, the Aculeata, in which the ovipositor was modified into a stinging device used to inject paralyzing venom into hosts or prey and to defend against enemies. The aculeate Hymenoptera include various parasitoids and cleptoparasites (such as cuckoo wasps, bethylids, tiphiids, scoliids, and mutillids), and several groups of predatory wasps (such as vespids, spider wasps, and spheciform wasps) that show variable nesting and social behavior (Gauld & Bolton 1988). It is within the aculeate Hymenoptera that ants and bees evolved. 24 Ward ES45CH02-Ward ARI 15 October 2014 8:51 The Closest Relatives of Ants There is compelling morphological and molecular evidence for the monophyly of ants (Brothers 1975, Bolton 2003, Brady et al. 2006, Moreau & Bell 2013), but until recently their relationship to other groups of aculeate Hymenoptera was unclear. The conventional view was that ants evolved from a nonsocial parasitoid wasp (Wheeler 1928, Wilson 1971, Dlussky 1983), perhaps similar in biology to the extant tiphioid wasp Methoca, which parasitizes tiger beetle larvae (Wilson et al. 1967). Morphological cladistic studies of the Aculeata by Brothers (1975, 1999) suggested that ants are sister to a clade comprised of two groups with contrasting biology: (a) scoliid wasps, which are ectoparasitoids on subterranean beetle larvae, and (b) vespids, which are mostly predatory nest- building wasps (such as potter wasps and yellow jackets), and include both solitary and eusocial species (Gauld & Bolton 1988). In some of the analyses by Brothers the ants were recovered as sister to the family Bradynobaenidae, a rather obscure group of wingless female wasps, whose biology is almost unknown but which are presumed to be parasitoids of other arthropods. Molecular phylogenetic analyses have produced contradictory conclusions about aculeate re- lationships, with ants being positioned as sister to apoids and scolioid wasps (Scoliidae and Bradynobaenidae) (Pilgrim et al. 2008, Debevec et al. 2012), sister to scolioid wasps only (Wilson et al. 2013), sister to a diverse group of mostly ectoparasitoid wasps in the tiphioid-pompiloid com- plex (Heraty et al. 2011), or sister to the small ectoparasitoid family Rhopalosomatidae (Klopfstein et al. 2013). These investigations have been based on a relatively small number (3–7) of gene frag- ments, with apparently limited power to recover some relationships within the Aculeata. A recent phylogenomic study ( Johnson et al. 2013) based on several hundred genes generated a well-resolved and strongly supported phylogeny of the major lineages of Aculeata (Figure 1). Among the primary conclusions are that vespid wasps are not closely related to ants or scoliids; bradynobaenids are nonmonophyletic, with true bradynobaenids being sister to scoliids and other bradynobaenid-like taxa being members of the tiphioid complex; and the closest relatives of ants are the spheciform wasps and bees, known collectively as the Apoidea ( Johnson et al. 2013). If these results hold up under more extensive taxon sampling (Danforth 2013), then they suggest that the early branching lineages of Apoidea, represented by solitary wasps such as mud daubers (Sphecidae) and cockroach wasps (Ampulicidae), might provide greater insight into the origin of ants than the ectoparasitoid wasps, such as scoliids, bradynobaenids, and tiphioids, to which ants were thought to be more closely related (Wilson 1971, Dlussky 1983, Brothers 1999). Comparisons with Apoidea Spheciform wasps capture and paralyze arthropod prey and transport the food items to a nest or cavity. Some species of the sphecid genus Chorion have parasitoid-like habits, abandoning the host Annu. Rev. Ecol. Evol. Syst. 2014.45:23-43. Downloaded from www.annualreviews.org Access provided by University of California - Davis on 11/25/14. For personal use only. after paralysis without placing it in a nest (Bohart & Menke 1976). This is presumably a secondary condition, but it does emphasize that there is a rather fluid boundary between parasitoid behavior and predation. Nest construction and food provisioning have long been considered important prerequisites for the evolution of eusociality, which is